Originally posted by haroon88 In low light, Sigma 17-50 at f2.8 does work better even at high ISO.
I don't think this has been brought up before, but there is something else that might be in play here in addition to the things that have been brought up before (which include that the Sigma 30 non-art is always soft off center, wide apertures will give a low in-focus area, slow shutter speeds will need a better technique to avoid motion blur even with SR, and higher ISOs will give you softer images, and Sigma 30mm lenses all usually have some autofocus issues, except maybe the very old screw drive ones).
This comment that I just quoted reminded me of my own battle with the Pentax shutter shake issues. The K-50 has some of it, but not as bad as my K-S1. The K-S1 is pretty bad in regards to that. The K-50, which I also own, has some as well.
The issue has been well documented in the forums. The K-1 also has it. The camera I started out with, the K20D, did not have any. My K10D doesn't either, but pretty much every camera since, with the possible exception of the K-3, has had it.
In the K-50 and K-S1, I get it from shutter speeds of about 1/60s to about 1/200s. The worse range is about 1/100s to 1/125s, it's very visible there. So in your examples posted, that would include pictures #2, #3 and #5. Pictures #1 and #4 should be free of shutter shake and might have other issues (#1 has a pretty slow shutter speed compared to the others but actually sort of looks the sharpest to me!)
There are two things that help control shutter shake: using a flash on to camera (the added weight/mass reduces the shake), or use a heavier lens - especially one with more weight at the front. The Sigma 17-50 2.8 is a pretty heavy lens compared to the 30 1.4, and is quite front heavy, so naturally it will reduce the shutter shake quite a bit.
I first noticed this back when I still has a K-r (which has a bit of shutter shake but also not quite as bad as the K-S1, which is probably the worst in the Pentax range in that regard). I was in my basement taking shots with my Tokina 19-35mm (a fairly large full frame wide angle zoom made of lots of glass and metal) and my Pentax DA 35mm f/2.4 "Plastic Fantastic" with both at 35mm and f4.5 (which is the longest focal length of the Tokina, and wide open!) and I could not figure out why the Tokina was consistently pin sharp but the super-lightweight DA 35 2.4 wasn't! And I used that DA 35mm on the K20D a lot prior to this and it was always pin sharp from f2.4. This bothered me for days if not weeks until I found out about shutter shake. And now I know that the featherweight DA 35 2.4 is the worse lens when it comes to shutter shake, it adds no mass nor weight to help it at all.
I would suggest that you do another shoot at night like the one you just did, set the camera to something like 1/25s or 1/30s in Tv mode, and then set your ISO range to top at ISO 1600 or even 800. Then let the camera select aperture. You might end up with some f/2 shots, and some f/4 or f/5.6. Just remember to use good technique - have your elbows brought in to touch your body for stability when shooting. With this you would bypass the shutter shake and the higher ISO issues and see if there are any issues left that related to the lens, AF or the K-50.